Download Systole and Diastole (Cardiac Cycle) The ventricles drive the blood

Systole and Diastole
(Cardiac Cycle)
The ventricles drive the blood in small volumes and synchronously into
pulmonary trunk and aorta. The contraction of the ventricular myocardium in
constantly repeated biphasic cardiac cycle, the contraction called systole;
relaxation called diastole (Fig. 5.10a, b). Each of these phases, systole
diastole, in turn divided into two phases:
the
this
the
and
Systole
 Contraction phase
 Ejection phase
Diastole
 Relaxation phase
 Filling phase
During the first part of systole, the ventricular myocardium begins to contract
(contraction phase). Because the atrioventricular valves are closed, and the
semilunar valves not yet open, the intraventricular pressure rises rapidly with no
change in volume (isovolumic contraction or isovolumetric contraction).
However, as soon as the pressure in the ventricles reaches the pressure in the aorta
(about 120 mmHg) or the pulmonary artery (about 20 mmHg), the semilunar valves
open, and the ejection phase begins. During this phase the ventricle is maximally
contracted, and a volume of 70 ml of blood (stroke volume) is ejected into the
arteries at rest. The intraventricular pressure then again falls below arterial pressure
and the semilunar valves close again.
Systole is followed by diastole, during which the myocardium relaxes, at first,
the atrioventricular valves remain closed, and the volume inside the ventricles
(intraventricular volume) is unchanged (the so-called end-diastolic volume of
about 70 ml). The pressure in the ventricles then falls below that in the atria, so that
the atrioventricular valves(AV) open and blood flows from the atria into the
ventricles (ventricular filling). The driving force for this movement is first of all the
beginning atrial contraction, and especially the descent of the base of the heart (Fig.
5.10a, b), by which the base approaches the apex during the ejection phase,
expanding the atria and thus sucking blood out of the veins. As the ventricular
myocardium relaxes, the base again travels upward, and blood reaches the ventricles
through the open atrioventricular valves.
Cardiac Output (CO)
Cardiac output is the volume of blood the heart pumps out, usually expressed in
liters per minute. The circulation volume (systemic and pulmonary circuit)
corresponds to the amount of blood put out by the heart per minute. The left and
right heart always moves equal amounts of blood, since otherwise the blood in one
circulation rapidly dammed up, while the other would suffer from lack of blood. The
cardiac output is determined by multiplying the heart rate (HR) the number of beats
per minute and stroke volume (SV) the blood volume ejected by each ventricle:
CO = HR x SV
If the heart at rest beats about 72 times per minute (pulse frequency) and each
contraction ejects about 70 ml of blood into the systemic circulation (stroke
volume), the calculated minute volume will be about 5 liters
Cardiovascular System Lecture No; 4
Dr. Abdul-Majeed Alsaffar
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CO=72 beats/min × 70 ml= 5.01 L / min.
This amount of blood roughly of person weighing 70 kg. During physical labor,
the muscles, among other organs, must be perfusing with more blood, and
circulating blood volume and blood pressure must increase correspondingly. Heart
rate and stroke volume can be raise to increase the circulating blood volume. In this
way, cardiac output can increase up to 25L /min during severe physical exertion,
that is, the normal blood volume can turn over about five times. Such an increase
might be achieved, for instance, if the stroke volume increases from 70 ml to 140ml
and the heart rate is briefly raised to 180 beats/min (180/min × 140 ml = 25,200
ml/min = 25.2L/min).
Intrinsic regulation
relationship
of
the
stroke
volume:
the
Frank-Starling
The amount of blood pumped by the heart each minute is determined almost
entirely by the rate of blood flow into the heart from the veins, which called venous
return, to the right atrium, then to ventricles as it does so, the pressure rises and
this stretches the ventricle. This myocardial fibers stretch, placing them under
degree of tension known as preload. The heart, in turn, automatically pumps this
incoming blood into the arteries, so that it can flow around the circuit again This
intrinsic ability of the heart to adapt for increasing volumes of blood inflowing known
as Frank-starling mechanism of the heart, (after the name of two great
physiologists).
Frank-starling mechanism: - means that the greater the heart muscle stretched
during filling, the greater the force of contraction and the greater the quantity of
blood pumped in the aorta. In other state (Within physiologic limits, the heart
pumps all blood that returns to it by the way of the veins).
Nerve Supply of the Heart
During hard work, the heart must eject up to five times more blood. This adaptation
of cardiac activity to increased demand partly achieved by the heart itself, but mainly
guided by the autonomic nerves to the heart.
The heart’s own
adaptation is achieved because greater filling stretches the cardiac muscle fibers.
The stretching leads to a stronger contraction and so to a greater stroke volume. By
way of the efferent sympathetic and parasympathetic nerves (Autonomic Nervous
System),the central nervous system (CNS) (vasomotor centers in the
brainstem) influences heart rate , excitability (bathmotropic effect), force of
myocardial contraction (inotropic effect), and impulse conductivity (dromotropic
effect).
The effect of the sympathetic system is to enhance cardiac activity. (e.g.
acceleration of heart rate, increase contractile force, acceleration of stimulus
conduction of the A-V node), while the parasympathetic system inhibits or
dampens cardiac activity (e. g., deceleration of heart rate, reduction in the force of
contraction, decrease in the rate of spread of a stimulus from the atria to the
ventricles). While activation of the sympathetic system occurs primarily through the
transmitter norepinephrine, the parasympathetic system (vagus nerve) exerts its
inhibitory action chiefly through the transmitter acetylcholine. The receptors for
Cardiovascular System Lecture No; 4
Dr. Abdul-Majeed Alsaffar
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norepinephrine on cardiac muscle are mainly beta-adrenergic. The hormone
epinephrine, from the adrenal medulla, combines with the same receptors as
norepinephrine and exerts the same actions on the heart. The receptors for
acetylcholine are of the muscarinic type.
The individual effects of the autonomic cardiac nerves expressed in different degrees,
since sympathetic and parasympathetic innervation differ in the various regions of
the heart. While the sympathetic innervation supplies the atria and ventricles
equally, the fibers of the vagus nerve (parasympathetic) run primarily to the atria
and to the sinus and A-V nodes.
Heart Sounds and Heart Murmurs
Two heart sounds, resulting from cardiac contraction, normally heard through a
stethoscope placed on the chest wall. The first sound, a soft low-pitched lub, is
associated with closure of the AV valves at the onset of systole; the second sound, a
louder dup, is associated with closure of the pulmonary and aortic valves at the
onset of diastole. These sounds, which result from vibrations caused by the closing
valves, are perfectly normal, but other sounds, known as:
Heart murmurs, are frequently a sign of heart disease. Murmurs produced by
heart defects that cause blood flow to be turbulent. Normally, blood flow through
valves and vessels is laminar; that is it flows in smooth concentric layers. Turbulent
flow can be caused by blood flowing rapidly in the usual direction through an
abnormally narrowed valve (stenosis), by blood flowing backward through a
damaged, leaky valve or cannot close competently (insufficiency), or by blood
flowing between the two atria or two ventricles through a small hole in the wall
separating them (called a septal defect).
The exact timing and location of the murmur provide the physician with a
powerful diagnostic clue. For example, a murmur heard throughout systole suggests
a stenotic pulmonary or aortic valve, an insufficient AV valve, or a hole in the
interventricular septum. In contrast, a murmur heard during diastole suggests a
stenotic AV valve or an insufficient pulmonary or aortic valve.
The type of valve disease can be determined by the intensity and timing of the
murmur in relation to the first or second heart sound at the point of auscultation.
The best place for auscultation (listening to), where the bloodstream is closest to the
chest wall after the closure of the respective valve (Fig.11): As the heart sounds
conducted along the bloodstream, the sounds auscultated where the valves project
to the surface of the thorax.
 Tricuspid valve, on right sternal border at the level of the 5th intercostals
space.
 Bicuspid or mitral valve, at the apex in the left 5 th intercostal space.
 Pulmonic valve, in the 2 nd intercostal space at the left sternal border.
 Aortic valve, in the 2 nd intercostal space at the right sternal border.
Cardiovascular System Lecture No; 4
Dr. Abdul-Majeed Alsaffar
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Fig. 11 Sites for cardiac auscultation. Red circles mark the sites for auscultation.
The
valves lie in a plane; the arrows indicate the direction of the bloodstream
(conduction of
the heart sounds).
References
1. Textbook of Medical Physiology 11th, Edition .by Guyton A.C.
2. Human Physiology The Basis of Medicine 2nd, Edition. by Gillian Pocock
and Christopher D.R.
3. Human Physiology: The Mechanism of Body Function 10th Edition. by
Vander, et al
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